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Todd Litman's avatar

Thanks David. You raise good points. However, I have a concern about the way you structure the analysis.

Like many travel demand studies it is based largely on commute mode share data, simply because that is the easiest data set to obtain. However, it is a poor indicator of travel activity, particularly active and micro modes (walking, bicycling, e-bikes and their variants). These statistics ignore non-commute trips, children’s travel, recreational travel, and active links of trips that include motor vehicle travel. For example, a bike-transit-walk trip is classified simply as a transit trip, and trips between parked vehicles and destinations are ignored even if they involve several blocks of walking.

The U.S. census reports that only 3.6% of commute trips are by active modes, which makes these modes seem insignificant, but the National Household Travel Survey, indicate that active modes serve a much more significant 12% of total trips, with higher rates in denser areas where traffic and parking problems are most severe, and various studies suggest that this share increases significant if communities invest a fair share of dollars and road space in serving those modes. In other words, there is significant latent demand for non-auto travel.

Improving active and public transport often leverages larger reductions in automobile travel, so each additional mile walked or biked reduces more than one vehicle-mile, for the following reasons:

• Shorter trips. Shorter active trips often substitutes for longer motorized trips, such as when people choose a local store rather than driving to more distant shops.

• Reduced chauffeuring. Better walking and bicycling conditions reduces the need to chauffeur non-drivers (special trips to transport a passenger). These often require empty backhauls (miles driven with no passenger). As a result, each mile of avoided chauffeuring often reduces two vehicle-miles.

• Increased public transit travel. Since most transit trips include walking and bicycling links, improving these modes supports public transit travel and transit-oriented development.

• Vehicle ownership reductions. Active mode improvements allow some households to reduce their vehicle ownership, which reduces vehicle trip generation, and therefore total vehicle-miles.

• Lower traffic speeds. Active travel improvements often involve traffic speed reductions. This makes non-auto travel more time-competitive with driving and reduces total automobile travel.

• More compact development. Walking and bicycling help create more compact, multimodal communities by reducing the amount of land needed for roads and parking, and creating more attractive streets.

• Social norms. As active travel increases, these modes become more socially acceptable.

This suggests that investments in non-auto modes can provide larger reductions in automobile travel than predicted by most modes that assume modest demand and one-to-one substitution effects.

For more information see:

Todd Litman (2022), "Fair Share Transportation Planning: Estimating Non-Auto Travel Demands and Optimal Infrastructure Investments," Victoria Transport Policy Institute (www.vtpi.org); at www.vtpi.org/fstp.pdf.

Todd Litman (2023), Comprehensive Transportation Emission Reduction Planning, Victoria Transport Policy Institute (www.vtpi.org); at www.vtpi.org/cterp.pdf.

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